Method of dissolving scale

ABSTRACT

A method of dissolving scale comprises contacting the scale with a solution of an alkali metal bisulfate. Another method of dissolving scale comprises contacting the scale with a solution of an alkali metal bisulfate and a strong mineral acid. A further method of dissolving scale comprises contacting the scale with a solution of an alkali metal bisulfate and an inorganic oxidizer.

BACKGROUND OF THE INVENTION

[0001] This invention relates in general to methods of removing scale,and in particular to chemical methods of dissolving scale to remove itfrom the interior surfaces of processing equipment.

[0002] In many industrial processes, the buildup of scale on theinterior surfaces of processing equipment is a significant problem.Materials such as alkaline metal salts precipitate out of the fluidsbeing processed and form adherent deposits or scale on the surfaces ofthe equipment. For example, scale can form on the surfaces ofevaporators, fermentors, and other equipment used in the manufacture ofethanol from corn. The buildup of scale can cause problems with heattransfer, fluid flow, and sanitation of the processing equipment.

[0003] Many different methods have been used for scale removal,including both physical and chemical removal methods. In the ethanolmanufacturing industry, various mineral acids have been employed todissolve the scale, sometimes in combination with caustic soda. To date,no universal and total solution to the scale problem has been uncovered.There is still a need for an effective and economical method of removingscale.

SUMMARY OF THE INVENTION

[0004] The present invention relates to a method of dissolving scale.For example, the method can be used to remove scale from an interiorsurface of processing equipment used for producing ethanol byfermentation of corn. The method comprises contacting the scale with asolution of an alkali metal bisulfate. In a preferred embodiment, thealkali metal bisulfate is sodium bisulfate. The scale is usually analkaline earth metal compound, such as calcium sulfate, calciumcarbonate, and/or calcium oxalate.

[0005] In another embodiment of the invention, the method comprisescontacting the scale with a solution of an alkali metal bisulfate and astrong mineral acid. In a preferred embodiment, the mineral acid isnitric acid. The solution preferably contains from about 1% to about 20%alkali metal bisulfate and from about 1% to about 10% nitric acid byweight. Preferably, the solution is substantially noncorrosive tostainless steel.

[0006] In a further embodiment of the invention, the method comprisescontacting the scale with a solution of an alkali metal bisulfate and aninorganic oxidizer. In a preferred embodiment, the inorganic oxidizer isammonium nitrate. Preferably, the solution contains from about 2% toabout 20% alkali metal bisulfate and from about 2% to about 20%inorganic oxidizer by weight.

[0007] Various advantages of this invention will become apparent tothose skilled in the art from the following detailed description of thepreferred embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] In the method of the invention, a scale is dissolved bycontacting it with a solution of an alkali metal bisulfate. Any type ofalkali metal bisulfate can be used in the method. Preferably, the alkalimetal bisulfate is sodium bisulfate, potassium bisulfate, or a mixturethereof.

[0009] The sodium bisulfate is a sodium salt of sulfuric acid generallyexpressed as NaHSO₄ (CAS Reg. No. 7681-38-1). It is also known as sodiumacid sulfate, sodium hydrogen sulfate, and bisulfate of soda. Apreferred sodium bisulfate is the food grade sodium acid sulfatemanufactured by Jones-Hamilton Co., Walbridge, Ohio. It has beencertified as GRAS and it meets Food Chemicals Codex, 4th EditionSpecifications. The sodium acid sulfate is a dry product comprisingcrystalline solid particles. The product includes sodium bisulfate in anamount of from about 91.5% to about 97.5% by weight and sodium sulfatein an amount of from about 2.5% to about 8.5% by weight.

[0010] The potassium bisulfate for use in the invention is a potassiumsalt of sulfuric acid generally expressed as KHSO₄ (CAS Reg. No.7646-93-7). It is also known as potassium acid sulfate and potassiumhydrogen sulfate.

[0011] The solution of the alkali metal bisulfate preferably containsfrom about 2% to about 50% alkali metal bisulfate by weight, morepreferably from about 5% to about 20%, and most preferably from about 5%to about 10%.

[0012] The remaining portion of the solution includes a solvent. Thesolvent is usually water, although any other suitable solvent can beused.

[0013] In one embodiment of the method, the solution includes a strongmineral acid in combination with the alkali metal bisulfate. The strongmineral acids are those which dissociate substantially completely inaqueous solution, such as nitric acid, hydrochloric acid, sulfuric acid,perchloric acid, hydrobromic acid, hydroiodic acid, chromic acid,permanganic acid, or mixtures thereof.

[0014] Preferably, the strong mineral acid is nitric acid. The solutionof the alkali metal bisulfate and the nitric acid preferably containsfrom about 1% to about 20% alkali metal bisulfate and from about 1% toabout 10% nitric acid by weight, more preferably from about 5% to about15% alkali metal bisulfate and from about 1% to about 5% nitric acid.

[0015] Preferably, the solution containing the alkali metal bisulfateand the nitric acid is substantially noncorrosive to the stainless steelused on the interior surfaces of processing equipment. The solutionpreferably has a corrosion rate of stainless steel of not more thanabout 0.025 inch/year, which is measured as described below inExperiment 3.

[0016] In another embodiment of the method, the solution includes aninorganic oxidizer in combination with the alkali metal bisulfate. Anysuitable inorganic oxidizer can be used. Preferably, the inorganicoxidizer is ammonium nitrate.

[0017] The solution of the alkali metal bisulfate and the inorganicoxidizer preferably contains from about 2% to about 20% alkali metalbisulfate and from about 2% to about 20% inorganic oxidizer by weight,and more preferably from about 3% to about 10% alkali metal bisulfateand from about 3% to about 10% inorganic oxidizer.

[0018] The scale dissolving method of the invention can be applied topractically any type of scale, including scales formed by deposition ofmaterials on surfaces, and scales formed by corrosion of surfaces. Inone embodiment, the scale is an alkaline earth metal compound,particularly a calcium compound. More particularly, the scale may becalcium sulfate, calcium carbonate, and/or calcium oxalate.

[0019] The method can also be used to dissolve scale in practically anytype of location, such as removing scale from practically any type ofsurface. In one embodiment, the method is used to remove scale from aninterior surface of processing equipment, e.g., equipment used forproducing ethanol by fermentation of grain (corn, barley or wheat). Suchequipment includes evaporators, fermentors, and other equipment known tothose in the industry.

[0020] The scale is contacted with the alkali metal bisulfate solutionfor a time and in a manner sufficient to dissolve the scale. The time ofcontact will depend on the type and amount of scale and the type ofsolution used. In some applications, the scale is contacted with thesolution for a time of at least about one hour, and preferably at leastabout two hours.

[0021] The scale can be contacted with the alkali metal bisulfatesolution by any suitable method, such as by spraying the solution on thescale, or by soaking the scale in the solution. The solution can also beadded to the materials being processed in processing equipment. In oneembodiment of the method, the solution is introduced into the interiorof processing equipment through a clean-in-place system connected to theequipment. The clean-in-place system usually includes a tank to hold thesolution, a pump, and connecting conduits between the tank and theequipment.

[0022] Experiment 1

[0023] Three samples of scale were obtained from various ethanol plants.A brown, flat scale was obtained. Two samples, one a chunk scale(evaporator) and the other an off white flat scale (fermentor), wereobtained from Commercial Alcohol Inc., Tiverton, Ontario, Canada.

[0024] In an effort to understand the effect of SBS concentration onscale removal, samples of the off-white fermentor scale were treated inthree concentrations of SBS solution. The samples were dried at 103° C.for 30 minutes, cooled in a desiccator for 30 minutes and thenimmediately weighed, then placed in solutions of 10%, 25% and 40% SBS(w/w) at 170° F. for 2 hours. At the end of the 2 hours the samples werecarefully dried, cooled and weighed. Table I summarizes the results ofthese tests. TABLE I SBS Concentration Study % Loss Sample Wt. SampleWt. Test Protocol Off-White Initial (g) Final (g) 10% SBS/2 Hours 52.3%0.0946 0.0451 25% SBS/2 Hours 17.2% 0.1323 0.1096 40% SBS/2 Hours  0.0%0.0924 0.0958 Ave. 23.2% 0.1064 0.0835

[0025] From the data in Table I we can see that the most effective SBSconcentration appears to be 10%. To confirm that this is a valid result,3 more samples of off-white scale were tested in 10% SBS. The sampleswere dried, cooled and weighed by the standard method, and treated at170° F. for 2 hours. The results in Table II prove to be consistent withthe initial results. In fact the 10% solution provided an average of65.6% in weight loss. TABLE II 10% SBS/Off-White Scale Test Protocol %Loss Sample Wt. Sample Wt. 10% SBS/2 Hours Off-White Initial (g) Final(g) Sample I 56.8% 0.1006 0.0435 Sample II 78.2% 0.1082 0.0236 SampleIII 61.9% 0.1505 0.0573 Ave. 65.6% 0.1198 0.0415

[0026] Table III summarizes the results for 5% SBS. Treating threesamples of off-white scale in a 5% SBS solution provided an average of69.9% weight loss. It appears that 5% SBS is the most of effective ofall the concentrations tested thus far. TABLE III 5% SBS/Off-White ScaleTest Protocol % Loss Sample Wt. Sample Wt. 5% SBS/2 Hours Off-WhiteInitial (g) Final (g) Sample I 66.6% 0.1080 0.0361 Sample II 70.6%0.1206 0.0355 Sample III 72.7% 0.1112 0.0304 Ave. 69.9% 0.1133 0.0340

[0027] Since 5% SBS seems to be the best operating concentration forthis test, the test was run on the other two scale sample types. Thesame drying and testing procedure was followed and the results of thesetests are summarized in Tables IV and V. TABLE IV 5% SBS/Brown ScaleTest Protocol % Loss Sample Wt. Sample Wt. 5% SBS/2 Hours Brown Initial(g) Final (g) Sample I 33.6% 0.2672 0.1774 Sample II 32.3% 0.3183 0.2124Sample III 36.6% 0.3089 0.1957 Ave. 34.2% 0.2966 0.1952

[0028] TABLE V 5% SBS/Chunk Scale Test Protocol % Loss Sample Wt. SampleWt. 5% SBS/2 Hours Chunk Initial (g) Final (g) Sample I 14.8% 0.62200.5300 Sample II 28.5% 0.4515 0.3228 Sample III 23.1% 0.5280 0.4060 Ave.22.1% 0.5338 0.4196

[0029] The testing performed on the chunk scale shows a range ofresults. The samples taken from the chunk scale were not consistent.Some had a larger crystal chunk, while others appeared to have a largerorganic build. The samples were selected to be as consistent aspossible, but it is not possible to get three identical samples.

[0030] It appears that the easiest scale to remove would be theoff-white, followed by the brown then the chunk. If 5-10% SBS solutionwas routinely injected into a clean plant, it is possible that the scalecould be removed as it forms, preventing the scale from building up.This would eliminate the need for a clean-in-place (CIP) system andreduce or eliminate the need for shutdown cleaning.

[0031] Experiment 2

[0032] Ethanol manufacturers would like to find a cost effective way toeliminate or reduce the buildup of scale in their plants. It is believedthat the scale is a combination of calcium sulfate, calcium carbonateand/or calcium oxalate (CaOx). We have proposed that sodium bisulfatecan dissolve all three forms of scale. Adding a solution to the productstream is not preferred because this would add additional water to theprocess. Alternatively, it was decided to test the effect of SBSdissolved in the thin stillage (the process stream immediately beforethe evaporator). A 5-gallon sample of stillage was obtained for testing.The stillage is high in solids, so the material must be filtered priorto use.

[0033] A piece of chunk scale (evaporator) was dried/weighed and dividedinto three samples. Each of these samples was subjected to 2 hours in a5% SBS solution made from filtered stillage. It should be noted that thepH of stillage as received is 4.15 and drops to 1.6 with addition of 5%SBS. As shown in Table VI, the average % loss was 21.1%, very close tothe average of 22.1% in 5% SBS (aq). This leads us to the conclusionthat reasonable results can be obtained using water or adding the SBSdirectly to the stillage. TABLE VI Chunk Scale in 5% SBS Diluted in ThinStillage Test Protocol % Loss Sample Wt. Sample Wt. 5% SBS/2 Hours ChunkInitial (g) Final (g) Sample I 22.3% 0.2462 0.1914 Sample II 24.7%0.4940 0.3720 Sample III 16.5% 0.3183 0.2659

[0034] To determine if this result would hold true for other scaletypes, three samples of brown scale were tested as above. As shown inTable VII, the % loss was 16% in 5% SBS/stillage as compared to betterthan 30% in 5% SBS (aq). The samples from this test were dried andsubjected to the conditions a second time in a freshly preparedsolution, and the sample again lost approximately 16%. The total %average loss for the two cycles was 29.5%, close to the average in 5%SBS (aq). These results suggest that an aqueous solution would performbetter than direct addition to thin stillage. TABLE VII Brown Scale in5% SBS Diluted in Thin Stillage % Loss Sample Wt. Sample Wt. BrownInitial (g) Final (g) Test Protocol 5% SBS/2 Hours Sample I 17.0% 0.46130.3828 Sample II 14.6% 0.4942 0.4220 Sample III 16.4% 0.4698 0.3926 Ave.16.0% (Second Pass) 5% SBS/2 Hours Sample I 16.1% 0.3842 0.3222 SampleII 11.5% 0.4230 0.3745 Sample III 12.9% 0.3935 0.3426 Ave. 13.5% TotalWeight Loss (Ave.) % 29.5%

[0035] In order to determine the effect of increased temperature for theability of SBS to remove scale, brown scale was treated in a 6% SBSsolution at 200° F. After 3 hours approximately one-half of the scalewas removed, as shown in Table VIII. TABLE VIII 6% SBS Boil Out SampleSample Test Protocol Wt. Wt. Time SBS 6%/200° F. % Loss Initial (g)Final (g) (hours) Brown Scale 40.7% 0.4135 0.2450 3 hr Brown Scale 45.7%0.4594 0.2496 3 hr Brown Scale 56.9% 0.3230 0.1392 3 hr

[0036] The ethanol industry would prefer not to add additional water totheir 10 process stream. Therefore, it would be advantageous to be ableto remove scale using the existing thin stillage stream as the diluent,instead of water. A sample of thin stillage was obtained from CommercialAlcohol Inc. and used to make a 6% SBS solution. The result was that 31%of the scale sample was removed in 5 hours.

[0037] Next, it was decided to try a blend of SBS and nitric acid forremoving scale. It was believed that nitric acid's oxidizing qualitieswould make it useful in removing scale, allowing it to oxidize theproteins in the organic material formed by the corn pulp in thestillage. It is this organic material that acts as a binder, making itmore difficult to remove the scale. Nitric acid is also used inpassivation, a process defined by ASTM A830 as “the removal of exogenousiron or iron compounds from the surface of stainless steel by means ofchemical dissolution, most typically by treatment with an acid solutionthat will remove the surface contamination, but will not significantlyaffect the stainless steel itself.” Furthermore they describepassivation as “the chemical treatment of stainless steel with a mildoxidant, such as a nitric acid solution, for the purpose of enhancingthe spontaneous formation of the protective passive film.” Not onlywould the nitric acid help to speed the scale removal without harmingthe stainless steel, it would actually protect the steel. A solution of6% SBS and 1% nitric acid was chosen as a starting point. The resultsare shown in Table IX. TABLE IX SBS/Nitric Acid Sample Sample TestProtocol Wt. Wt. Time 6% SBS/1% Nitric % Loss Initial (g) Final (g)(hours) Brown Scale 94.6% 0.2353  0.0127 6.0 hr Chunk Scale 53.7%0.3863  0.1790 6.5 hr Brown Scale 66.8% 0.2725  0.0905 5.5 hr BrownScale 56.2% 0.3195  0.1399 5.5 hr Brown Scale 67.8% 0.24742 0.0796 5.5hr Brown Scale 63.0% 0.57020 0.2108 6.0 hr

[0038] The SBS/nitric blend showed very good results with over 90%removal of brown scale in initial tests. Further tests showed goodresults with over 60% removed in about 6 hours. Even chunk scale showeda substantial removal of 53.7% in over 6 hours.

[0039] This protocol using SBS and nitric acid was next tested in thinstillage. A new sample of thin stillage was obtained from Ethanol 2000,Bingham Lake, Minn. The starting pH of this stillage was 3.5, more than0.5 lower than the stillage from Commercial Alcohol Inc. Table X showsthat the SBS/nitric acid blend removed approximately 18-30% of bothchunk and brown scale in 5-6 hours. The removal of scale was notincreased if the stillage was filtered. It was theorized that thepresence of the organic corn material might act as a buffer, thusreducing the ability of the acids to remove the scale. The results forSBS/nitric in thin stillage were similar to 6% SBS alone in thinstillage. TABLE X SBS/Nitric Versus SBS in Thin Stillage Test ProtocolSample Sample 6% SBS/Thin Stillage Wt. Wt. Time (Filtered) % LossInitial (g) Final (g) (hours) Chunk Scale 18.6% 0.5570 0.4532 5.0 hrBrown Scale 25.4% 0.4687 0.3498 5.0 hr 6% SBS/Thin Stillage (Unfiltered)Brown Scale 31.1% 0.4208 0.2898 6.5 hr

[0040] Due to the suspected ‘buffering’ action of thin stillage, testswere performed with increased SBS (10%) and nitric acid (2%). Brownscale was used and the tests were carried out in 3 different media:water and thin stillage (filtered and unfiltered). The tests in thinstillage were about one-half as effective as the test in water. Theresults are summarized in Table XI. TABLE XI 10% SBS/2% Nitric Acid inWater and Thin Stillage Sample Sample Test Protocol % Loss Wt. Wt. Time10% SBS/2% Nitric Acid Brown Initial (g) Final (g) (hours) Water 92.0%0.2712 0.0217 8.0 hr Stillage, Filtered 59.9% 0.2863 0.1147 8.0 hrStillage, Unfiltered 49.3% 0.3046 0.1544 7.0 hr

[0041] The results indicate that a clean-in-place system may be a betteroption than adding the SBS/nitric blend to the thin stillage. It canalso be concluded that 5-10% SBS in combination with 1-2% nitric acid isvery effective at removing scale.

[0042] Nitric acid is a strong oxidizing acid. A dry, inorganic oxidizerwas tested in place of nitric acid to determine its effectiveness inoxidizing the organic material. Table XII shows that ammonium nitrate isjust as effective as nitric acid in removing scale. Ammonium nitrate isdry and can be easily blended and stored with the alkali metalbisulfate. Additionally, ammonium nitrate is not corrosive to steel,including carbon steel. TABLE XII 10% SBS/2% Ammonium Nitrate TestProtocol Sample Sample 10% SBS/2% Weight Weight Ammonium Nitrate % lossinitial (g) final (g) Time (hours) Brown Scale 74.0 0.1914 0.0498 8.0 hr

[0043] Experiment 3

[0044] The method of the invention can be employed in ethanol plants toremove scale buildup in process evaporator tubes and other processequipment. Most evaporators are made of stainless steel material, with304 and 316 the most common alloy used. Previous work showed that SBS isnoncorrosive on these steel alloys. An experiment was done to determinethe corrosivity of an SBS/nitric acid blend on the same alloys.

[0045] A solution consisting of 10% SBS and 2% nitric acid was prepared,and divided equally among 4 beakers. The beakers were placed on a hotplate and the solutions heated to 170° F. A sample of 304SS was weighedand placed into 2 of the beakers and a sample of 316SS was weighed andplaced into the other 2 beakers. The beakers were loosely covered toreduce evaporation over the test period. The metal was treated in thissolution for a period of 48 hours. At the end of the test period thesamples were rinsed, dried and reweighed to determine weight loss andcorrosion rate. The data are summarized in Table XIII. TABLE XIIICorrosion Data for 304 and 316 Stainless Steels Weight Corrosion Alloy %Loss Initial Weight Final Weight Loss (g) Rate (in/yr) 316ss 0.1528.4614 28.4180 0.0434 0.01727 316ss 0.13 28.2787 28.2432 0.0355 0.01413304ss 0.05 29.9696 29.9547 0.0149 0.00569 304ss 0.05 29.7082 29.69460.0136 0.00519

[0046] The data show a minimal amount of weight loss over the 48-hourtest period. A typical clean-in-place procedure would take approximately8-10 hours, and therefore should show lower corrosion rates than thoselisted in Table XIII. The corrosion rate for 316ss is approximatelythree times higher than for 304; however, it is still low at an averagerate of 0.015 inch per year. The results of these tests confirm that useof an SBS/nitric acid cleaning solution should not be significantlycorrosive on stainless steel when used to remove calcium scale.

[0047] The principle and mode of operation of this invention have beenexplained in its preferred embodiments. However, it must be understoodthat this invention may be practiced otherwise than as specificallyexplained without departing from its spirit or scope.

What is claimed is:
 1. A method of dissolving scale comprisingcontacting the scale with a solution of an alkali metal bisulfate.
 2. Amethod according to claim 1 wherein the solution contains from about 2%to about 50% alkali metal bisulfate by weight.
 3. A method according toclaim 1 wherein the alkali metal bisulfate is sodium bisulfate.
 4. Amethod according to claim 1 wherein the scale is an alkaline earth metalcompound.
 5. A method according to claim 4 wherein the scale is selectedfrom the group consisting of calcium sulfate, calcium carbonate, calciumoxalate, and mixtures thereof.
 6. A method according to claim 1 whichcomprises removing scale from an interior surface of processingequipment used for producing ethanol by fermentation of grain.
 7. Amethod according to claim 1 which comprises removing scale from aninterior surface of processing equipment by introducing the solutionthrough a clean-in-place system connected to the processing equipment.8. A method of dissolving scale comprising contacting the scale with asolution of an alkali metal bisulfate and a strong mineral acid.
 9. Amethod according to claim 8 wherein the mineral acid is nitric acid. 10.A method according to claim 9 wherein the solution contains from about1% to about 20% alkali metal bisulfate and from about 1% to about 10%nitric acid by weight.
 11. A method according to claim 8 wherein thesolution is substantially noncorrosive to stainless steel as measured bya corrosion rate of not more than about 0.025 inch/year.
 12. A methodaccording to claim 8 wherein the alkali metal bisulfate is sodiumbisulfate.
 13. A method according to claim 8 wherein the scale is analkaline earth metal compound.
 14. A method according to claim 13wherein the scale is selected from the group consisting of calciumsulfate, calcium carbonate, calcium oxalate, and mixtures thereof.
 15. Amethod according to claim 8 which comprises removing scale from aninterior surface of processing equipment used for producing ethanol byfermentation of grain.
 16. A method according to claim 8 which comprisesremoving scale from an interior surface of processing equipment byintroducing the solution through a clean-in-place system connected tothe processing equipment.
 17. A method of dissolving scale comprisingcontacting the scale with a solution of an alkali metal bisulfate and ainorganic oxidizer.
 18. A method according to claim 16 wherein theinorganic oxidizer is ammonium nitrate.
 19. A method according to claim16 wherein the alkali metal bisulfate is sodium bisulfate.
 20. A methodaccording to claim 16 wherein the solution contains from about 2% toabout 20% alkali metal bisulfate and from about 2% to about 20%inorganic oxidizer by weight.